Hot melt adhesives are bonding compositions which achieve a solid state and resultant strength by cooling as contrasted with other adhesives, such as emulsion or lacquer adhesives, which achieve the solid state through evaporation or removal of solvent. A hot melt adhesive is normally a solid thermoplastic material. The application of heat melts the solid hot melt adhesive and after the heat source is removed, it solidifies simply by cooling. This is in direct contrast to thermosetting adhesives which set or harden by the application of heat and which remain hard upon continued application of heat.
Hot melt adhesives are widely used in packaging, shoe manufacturing, and other industries and are particularly useful because of their tackiness in the molten state and the speed with they form strong bonds. However, utilization of these combinations is, at times, limited because of certain undesirable properties which adversely affect their overall performance. For example, high melt viscosity makes the adhesives hard to apply to the surface of the substrate, and poor heat stability results in thermal degradation.
In order to yield satisfactory results in commercial usage, hot melt adhesive compositions must possess several other properties, in addition to the necessary degree of tackiness at the application temperature. In particular, these compositions must display a low viscosity at the application temperature so as to easily wet the surface of the substrate and also possess heat stability to minimize molecular degradation due to prolonged heating, oxidation, and hydrolysis.
It is important for hot melt adhesives which are applied to heat-sensitive materials, such as textiles, to have relatively low melting points. However, an excessively low viscosity in the melt is disadvantageous in textile lamination. In this case, the object is to bond a facing material to a lining material such that the laminant will retain the textile character. Fusion adhesives in powder form are preferred in textile lamination. Such powders are sintered onto the lining material, usually woven fabrics, by means of special applicator systems, and then cemented to the facing materials with the application of heat and pressure. Hot melt adhesives which are highly resistant to dry cleaning fluids are also needed in such textile applications.